1. Field
Embodiments of the present invention relate to a phase current prediction method to measure DC link current of an inverter through a single current sensor provided between the inverter, which drives a motor, and a DC voltage source to predict phase current of the motor.
2. Description of the Related Art
Generally, an inverter circuit is a power conversion device that converts direct current (DC) power into pulse-shaped 3-phase (U, V and W) alternating current (AC) power having a variable frequency. Due to reduction in energy consumption and easiness of output control, the inverter circuit has been increasingly used to drive motors used in electric products such as a washing machine, refrigerator, and air conditioner.
In order to properly control a motor using the inverter circuit, a method has been used which detects phase current applied to the motor and controls current to be applied to the motor based on the detected phase current in a pulse width modulation (PWM) scheme.
Recently, a method has been widely used which introduces a space vector concept to control a motor in a space vector pulse width modulation (SVPWM) scheme. This method controls current to be applied to the motor based on phase current detected by a single current sensor.
In the method that controls the motor in the SVPWM scheme, sampling-based average current is used, whereas current actually flowing to the motor continuously varies, and is not fixed, as shown in FIG. 1.
In an SVPWM scheme using three current sensors, carrier start or middle values iam, ibm and icm, which are control representative values indicative of average current in a PWM period, are measured for control of a motor, the measured values are determined to be phase currents, and currents to be applied to the motor are controlled based on the determined phase currents.
However, in the SVPWM scheme using the single current sensor, current is measured in a switching period, not at the middle of a sampling period, resulting in occurrence of a current error.
As shown in FIG. 1, current measurements are made in effective voltage vector periods, and the resulting values exhibit differences with iam, ibm and icm, which are representative values of those periods. For example, in the case where the angle of an output voltage is present between 0 and π/3, current ias and current ics are measured respectively at Ta and Tc in a scheme that measures current flowing to a DC link. In this case, there are current errors Δiam and Δicm between ias and ics and iam and icm as in the following equations 1 and 2.Δiam=ias−iam  [Equation 1]Δicm=ics−icm  [Equation 2]
These current errors continuously vary with the operation state of the motor, resulting in occurrence of a current ripple. This current ripple causes noise in the motor and system, thus degrading a product quality.